Steep slopes, embankments and subgrades of earth often require stabilization to prevent catastrophic soil movement. Generally, soil stabilization is required in construction involving roadways, foundations, retaining walls, and the like, in which slopes, embankments, and subgrades of soil are susceptible to soil movement. While stabilization can be accomplished by using high quality, select soils in the slopes or subgrades, it is often desired to reuse existing soil at construction sites. In such cases, and sometimes even with use of supplemental select soils, acceptable safety factors require the construction of additional structures to effect stabilization of the soil in the earthen structure. Some soil stabilization applications use underlayments or layers of sheet materials which are covered with backfill materials, while other applications incorporate retaining walls from which sheet materials extend and are covered with backfill materials. The retaining walls typically are constructed of a plurality of blocks which connect together. Some known blocks have bores which receive pins or dowels to connect blocks in vertically adjacent tiers. Other types of blocks have opposing top and bottom surfaces which are often configured for interlocking engagement, in order for the wall made of the blocks to be mechanically connected together.
These retaining walls also generally include at least one laterally extending horizontal reinforcing sheet that prevents sliding or rotational failure of the slope. In a typical site construction, the retaining wall includes many vertically-spaced sheets. A side portion of the sheet attaches to the wall, such as by being held between adjacent tiers of blocks or by connectors disposed in the wall.
Generally, the sheets are substantially flat sheets which define a plurality of large openings or apertures. During construction of the wall, backfill covers the sheet. Rocks and stone, generally known as gravel, and soil in the backfill occupy apertures in the sheets. Gravel generally is a material of which 50% or more is retained on a number 4 sieve (4.75 mm openings). The occupancy of the apertures by backfill is known in the industry as "strike-through." The apertures permit strike-through of backfill materials from one side of the sheet to the other, which is a desirable feature for soil stabilization. The strike-through materials mechanically connect the sheet to the backfill, and thereby secure the retaining wall to the backfill. Such sheets and backfill are also used in underlayments for roadways and foundations or in layers for reinforcement of steep embankments and slopes.
The anchorage or pullout resistance of a sheet in backfill is the result of the following separate mechanisms. One mechanism is the shear strength along the top and bottom of the load-bearing elements of the sheet. A second of the shear strength mechanisms is the contribution along the top and bottom elements of the sheet transverse to the load-bearing elements. For those sheets where strike-through occurs, a third mechanism provides passive resistance of the backfill against the front of the transverse elements. The front portions of the striking material makes contact with the front face of the transverse elements. The resistance loading is transferred to the intersection or junction of the longitudinal and transverse elements. The intersection transfers the load to the load-bearing elements.
Several types of sheets have been used for stabilizing earthen slopes and subgrades. The sheets are generally woven, knitted, or stitch-bonded textiles or extruded, oriented plastic sheets.
Woven, knitted, or stitch-bonded textiles have longitudinal yarns (warp yarns), interwoven, knitted, or stitch-bonded with transverse yarns (weft yarns). These textiles are characterized as having poorly defined and dimensionally unstable intersections or junctions between the warp and weft yarns. The large surface area of textile sheets generally is substantially closed, and this prevents passage of the soil backfill through the sheet during installation and compaction of backfill. Without significant amounts of soil striking through open portions of the sheet, the sheet has reduced anchorage strength or reduced resistance to pullout. Woven, knitted, or stitch-bonded textiles exhibit generally poor abrasion resistance and are easily damaged during installation. When such textiles are placed under a load, the yarns transverse to the loading tend to slide relative to the yarns parallel to the loading. The intersection defined by the warp and weft yarns become distorted. The shifting of the yarns and induced soil movement reduces the shear orientation of the soil. This reduces the shear strength contribution along the top and bottom of the sheet. Further, because the aforementioned textiles are substantially or entirely closed, there is little, if any, contribution of the passive resistance mechanism discussed above, to the anchorage or pullout resistance of this sheet.
Increased junction strength at the intersection of the warp and weft yarns overcomes the tendency of the yarns to slide or shift. This may be accomplished by coating the sheets to provide a stronger junction between the warp and weft yarns at the intersections and also to an extent by the particular weave pattern. However, when a woven, knitted, or stitch-bonded textile without well defined openings is coated, it generally becomes impermeable. An impermeable textile will result in significantly reduced drainage of surface and ground water vertically through the reinforced soil structure. Without drainage, destabilizing hydrostatic pressures will develop within the soil structure.
Another type of sheet for stabilizing earthen slopes and subgrades is extruded geogrid sheet formed with flexible, high strength oriented polymer plastics. The sheets are generally substantially flat sheets with relative large openings of 12 mm or larger. The openings, generally known in the industry as "apertures," are defined by longitudinal ribs and transverse bars. The sheets typically have relatively even ratios of open apertures and closed space defined by the ribs and bars. The backfill of gravel and soil readily strikes through the apertures and the gravel in the backfill forms mechanical linkages between the backfill and the geogrid.
While extruded geogrids have been gainfully employed, there are drawbacks which limit their use in certain applications. Geogrid installations are significantly more expensive in materials and labor to install. Generally, the polymeric extruded/oriented geogrids are most effective when using gravel as a backfill. Often, however, the backfill for a site is comprised primarily of earthen soil materials removed from an excavation, with supplemental fill dirt. These materials, however, being substantially smaller than the apertures in the geogrid, fail to satisfactorily mechanically engage with the geogrid.
Additionally, extruded geogrids have thick transverse bars and thin longitudinal ribs. The thin ribs are oriented. The transverse bars and the junction between the longitudinal ribs and the transverse bars are not oriented. Therefore they have lower tensile strength and modulus. This gives inconsistent tensile and elongation properties when the longitudinal ribs are placed under load. The extruded geogrids are also heavy and awkward to maneuver, and often, mechanical devices are required to hold the geogrid during installation. For example, extruded geogrids tend to have high memory. The geogrid typically is supplied in rolls, and the geogrid tends to re-roll during installation. The geogrid accordingly requires firm holding during installation.
Another type of large aperture geogrid has addressed these problems. This type is a coated textile made of woven, knitted or stitch-bonded yarns. The textile is coated with a curable material to form stronger junction intersections than is provided by noncoated textiles. These types of geogrids define apertures of relatively large sizes having dimensions of 12 mm (1/2 inch) or larger, designed to replicate the typical dimensions of extruded, oriented polymer plastic geogrids. These are generally lighter-weight than extruded, oriented polymer geogrids, which facilitates handling and installation. The relatively large surface area of this type of textile sheet provides reasonably high shear stress when subjected to normal stress. However, coated large aperture geogrids do not have the junction strength of extruded, oriented polymer plastic geogrids. Due to the long distance between transverse elements and the resulting high load applied by the passive resistance of the strike-through materials on the transverse elements, the coating is often not strong enough to maintain secure junctions. In addition, the long distance between junction joints and the very high flexibility of the yarns comprising the longitudinal and transverse elements contributes to significant deformations of the transverse elements and junctions under load and results in potential movement of the geogrid in the direction of the applied load within the soil structure.
The large apertures in such geogrids provide linkage between the geogrid and the gravel and/or course grained soil in the backfill. However, many constructions, such as steep slopes, retaining walls, and embankments over soft subgrades, use backfill that is typically only or primarily soils of small particle size. "Small particles" in soil are those in which more than 50% pass a number 4 sieve. Such small soil particles insufficiently mechanically transmit bearing loads against the large widely spaced transverse elements that define the large apertures in geogrids previously used. Large aperture grids, while used in such applications, are most efficient when less than 50% of the soil particles pass a number 4 sieve.
Accordingly, there is a need in the art for small-aperture coated textile sheets to mechanically stabilize slopes, embankments, subgrades, foundations, and retaining walls with backfill materials of primarily soils of small particles. It is to such that the present invention is directed.